1. Field of the Invention
The present invention relates to a method and apparatus that enables measurement of the three-dimensional distribution of an electric field in microdomains.
2. Description of the Prior Art
Techniques that are used in ultrahigh-speed integrated circuit (IC) development for measuring the electric fields in microdomains include electro-optic sampling (EOS), in which a high-sensitivity electro-optic crystal is brought into proximity to the IC surface to detect fringe electric fields from circuit interconnections.
A detailed description of the electro-optic sampling principle is provided by Shintaro Miyazawa, in Advanced Electronics Series, I-14, Category I: Electronics Materials, Properties, Devices, Optical Crystals, pp 96-101. FIG. 9 illustrates the basic principle of electric field measurement using an electro-optic crystal. With reference to FIG. 9(a), when an electro-optic crystal 102 disposed at a probe tip is brought into proximity with an IC substrate 101 to be measured, a fringe electric field 11l leaking into space as a result of a signal in an interconnection 103 or the substrate induces a change in the refractive index of the electro-optic crystal 102, due to the electro-optical effect, with the amount of the change corresponding to the fringe electric field 111. The end face of the electro-optic crystal 102 facing the IC substrate 101 is provided with a dielectric mirror 104 having a high reflection factor. A pulse laser 110 provides the probe beam. The beam, affected by the refractive index of the electro-optic crystal 102, is reflected back in a polarized state corresponding to the refractive index change, and the change in the polarization state of the reflected beam is detected by passing the beam through a polarizing plate (not shown). The electro-optic crystal 102 is maintained by a support 105. With respect to FIG. 9(b), when an electric signal is applied to the interconnection 103, changing the timing of the laser beam relative to the signal enables signal changes in the interconnection to be detected as changes in optical intensity and reproduced. This detection method is known as sampling, a method of sampling the signal amplitude at the same intervals as the pulses. Therefore, with respect to the waveform, the laser beam applied to the object to be measured must be applied as a periodically, repeating signal. The sampling temporal resolution of the pulse laser beam is determined mainly by the pulse width of the beam and the time it takes to pass through a crystal domain where there is an electric field. The response time, which is the time it takes for the double refraction factor to be changed by the electric field, is 100 fsec, which is short enough that it can be ignored. It is considered readily possible to attain a one-picosecond-level response time.
With reference to FIG. 10(a), when the pulse laser 110 irradiates the electro-optic crystal 102 over the interconnection 103, the electro-optic crystal 102 is affected by a vertical leakage electric field (vertical field probing). When the electro-optic crystal 102 irradiated by the pulse laser 110 spans interconnections 103, as in FIG. 10(b), the electro-optic crystal 102 is affected by a horizontal electric field (horizontal field probing). It is therefore necessary to select the electro-optic crystal according to the detected electric field. Table 1 shows the properties of electro-optic crystals used in the EOS methods of the prior art.
TABLE 1RefractiveElectro-opticPermit-Wave-IndexConstanttivitylengthDetectedCrystalnRIJ (pm/V)ε(≧μm)FieldLiNbO32.2330.8 (33)320.4HorizontalVertical(55°Cutoff)LiTaO32.1430.3 (33)430.4HorizontalKDP1.5110 {grave over ( )}6 (63)480.2VerticalKTiOPO41.833515.40.35HorizontalKTiOAsO41.8  40 (33)18Vertical(11°Cutoff)Bi12SiO202.5 5.0 (41)560.4Bi12GeO202.1 1.0 (41)160.5VerticalBi12TiO202.565.75 (41)0.45VerticalGaAs3.5 1.4 (12)120.9VerticalZnTe3.1 4.3 (12)100.6VerticalCdTe2.8 6.8 (12)9.40.9Vertical
Generally, horizontal field probing is applicable to coplanar ICs, such as MMICs and ICs with few adjacent interconnections, but in the case of conventional ICs, spatial resolution is degraded by optical crosstalk and the like. Conversely, vertical field probing is sensitive to electric fields perpendicular to the IC surface, limiting application to the interconnections themselves, but provides good spatial resolution. Oxide crystals include KDP, KTiOPO4, LiNbO3 or other such ferroelectric electro-optic crystals, and compound semiconductors, such as GaAs and CdTe. Here, the crystal's permittivity becomes the capacitive load of the object to be measured. However, this load can be reduced when the distance between the probe and the IC surface is set to be as large as around 2 μm. Regarding spatial resolution, a probe-tip beam diameter of around 2 μm is considered to be the current limit.
The above explanation has been made with respect to a single probe sensor portion at the probe tip. However, U.S. Pat. No. 5,991,036 discloses an EOS apparatus in which a plurality of sensor portions are arranged in two dimensions, and electromagnetic radiation is processed to produce an image. In this disclosure, however, the electro-optic crystal layers of the sensor portions are spaced equidistantly from the object to be measured.
As described in the foregoing, the electro-optic sampling method of the prior art is used for point-by-point measurement, in three-dimensional space, of the electric field around the object to be measured. Therefore, when it is desired to obtain a three-dimensional image of the electric field distribution, the requisite measurements take a long time. In the case of the EOS apparatus that uses a plurality of sensor portions arranged in two dimensions and processes electromagnetic radiation to obtain an image, it is difficult simultaneously to measure the electric field strength that is dependent on the distance from the object to be measured.
An object of the present invention is to provide a method and apparatus for three-dimensional measurement of electric field distribution that enables rapid measurement and imaging of the three-dimensional distribution of the electric field in microdomains.